1. Field of the Invention
The present invention relates to a method and apparatus for cleaning a cylinder of a rotary printing machine, and more particularly, to a method and apparatus for cleaning printing cylinders, including blanket cylinders and back pressure cylinders as well as plate cylinders or form cylinders, for example, in a varnishing unit.
2. Discussion of the Related Art
A method for cleaning a plate cylinder is disclosed in DE-B 1 808 909. In this disclosure, in order to achieve a wiping effect for removing soil and other deposits such as ink, varnish residues, and dust on the plate cylinder, a roller which rests against the plate cylinder is driven at a different radial speed from that of the plate cylinder.
A washing device which is mounted in a rotary joint and which has a washing roller is disclosed in EP 0,004,605 A1. The washing device has a control cam in the region of a gripper of an impression cylinder. By means of this control cam, the washing roller which is engaged against the impression cylinder is lifted up from the outer surface in an angular motion so that it does not collide with the grippers.
It is also known from EP 0,548,500 A1 that an accumulation of fluid, consisting of detergent and removed soils, such as ink and dust, forms at the point of contact between the washing roller and the impression cylinder. The accumulation of fluid at this point of contact is pushed along in front of the washing roller in the direction of rotation and, when passing through the cylinder gap, is conveyed into the cylinder gap, which leads to the reduction or elimination of the accumulation of fluid. In this case, the washing roller can be pressed against the outer surface of the cylinder with variable force.
The above described cleaning methods and devices have certain disadvantages associated therewith in that the hydrodynamic conditions and the frictional engagement at the point of contact between the washing roller of the washing device and the particular cylinder to be cleaned, are given too little attention. Thus, at the end of the cleaning operation, there is often a residual layer of fluid over the entire outer surface of the cylinder, i.e., the outer surface of the cylinder is not yet sufficiently dry when subsequent printing begins. This situation occurs, in particular, in the case of detergents which do not contain rapidly evaporating solvents, for example, detergents formed from a vegetable base. Furthermore, there is the risk that detergents or even water may pass into the cylinder gap due to the spraying effect of nozzles utilized to deliver the detergents and water or even due to the washing device itself.
The transporting of excess detergent (including soil and other deposits) into the cylinder gap in accordance with EP 0,548,500 A1 also has a disadvantage associated therewith in that the printing quality of the sheets in the subsequent printing cycle is lowered. The excess detergent in combination with the removed soils and other deposits forms a sludge which collects in the cylinder gap with the heaviest concentrations collecting at the edge of the cylinder gap which is arranged at the beginning in relation to the direction of rotation of the printing machine. The use of excess detergent increases the consumption of detergent unnecessarily without improving the quality of the cleaning job itself. Additionally, an uncontrolled supply of detergent can lead to the washing device floating, e.g., hydroplaning, on the outer surface of the cylinder thereby inhibiting efficient cleaning.
A blanket cylinder which has flattened portions in the cylinder gap on both sides of the gap edges is disclosed in DE 2 613 687 B2. A cylinder of this type poses certain unique cleaning difficulties for known washing devices.